Rivulus: From Boring to Beautiful

Author: Thomas R. Grady

Photographer: Frans Vermeulen

Once dismissed as drab brown fish, rivulus killifish now are known to provide a wealth of vibrant colors and interesting behaviors to their keepers.

Long known only as ugly brown fish with very few fans, today rivulus are gaining supporters throughout the hobby. In fact, some species are among the most distinctive and beautiful killies available. Others lead fascinating lives that we are only just beginning to understand. New species are discovered every year, with international collectors visiting South and Central America and coming home with a wide variety of beautiful and challenging fish.

A yet-unnamed species designated as Rivulus sp. Peru 05-260 is a true challenge to breed but a very long-lived killie—the male in one of my tanks is currently over five years old and doesn’t look a day over three. Then there is R. mahdiaensis, perhaps the most beautiful of all, but certainly challenged by R. xiphidius, R. frenatus, and R. christinae.

Natural History

Rivulus live in all types of conditions in nature. They are found in salt/brackish mangrove swamps, fast-moving streams, and softwater rainforest rivers over a huge portion of South and Central America and the Caribbean Basin, and they even extend into the lower United States.

Care and Maintenance

The overall care and breeding of killifish in general holds true for most rivulus. The vast majority are mop-spawners, and most lay their eggs near the surface of the water. You absolutely must cover the tank tightly. These fish are notorious jumpers that will inevitably and unerringly find their way through the smallest hole.

Most Rivulus species adapt easily to any water conditions. In addition, rivs are the toughest fish I have ever owned. Here in the winter storm–filled boondocks of the Adirondack Mountains in northern New York, I’ve never lost a rivulus to low temperatures, despite the fact that in the ice storm of 1998 I literally saw thin ice form on the top of many tanks. My R. cryptocallus and R. hartii (both Caribbean species) survived to continue breeding as soon as it warmed up. That week-long power outage devastated the rest of my fishroom.

My water is a bit on the hard side (200 ppm) with a pH of 7.5 to 8.0. Though the natural habitats of many Rivulus species are quite different (soft and acidic), most adapt well to my water.

They are comfortable in a smaller tank in pairs or small groups. There can be some aggression, but most are peaceful. I have a male R. sp. PAN 05-18 with three females in a 2½-gallon tank. They are slightly larger than 3 inches and produce eggs regularly despite limited water changes.

The majority of my tanks are simple setups. I do not use a filtration system or air supply. Instead I add a box filter filled with lava rock pieces to all my tanks, but I do not run air through them. The tank is heavily planted, primarily focusing on floating plants like water sprite, Riccia, hornwort, and (unfortunately) the ever-present duckweed.

Recently, thanks to fellow hobbyist Gary Greenwood, I discovered a great peat fiber. Many of my tanks are now maintained with a thick carpet of this substrate. Regular water changes are mandatory, as the peat fibers degrade rapidly. I do not do much other maintenance on the tanks. Many have heavy algae growth on the sides and all have spawning mops.

With over 200 tanks in my fishroom I can’t manage weekly water changes, but at the very minimum every tank gets a 50-percent water change once a month and tanks with the peat fiber get a water change every two weeks.

I do collect rainwater and use a carbon filter on it. In my area acid rain is prevalent—with the pH being as low as 3—so I have to be careful. I use rainwater if I find that my fish are not producing as expected. For the most part this will sparkspawning.

Feeding Rivulus

Killifish are generally picky eaters that often require live foods, but while rivulus love live foods, many are not finicky and will do quite well on dry foods. My personal preference, though, is to use a variety of foods and feed two to three times daily. I culture whiteworms, daphnia, Grindal worms, and baby brine shrimp. I feed live worms and daphnia on alternate days as the first course, with frozen foods as the second course and baby brine shrimp as dessert. I rarely feed dry foods unless my live cultures are down and I am out of frozen food. If I had an easy source of blackworms, I would add them to the rotation. [Check out Tom’s methods of culturing live foods at www.tfhmagazine.com/blogs.]

Breeding

Any advice you find about mop-spawning killies will probably work fine for Rivulus species. Spawning mops used by the majority of killie keepers are acrylic yarn bundles. Recently I have taken to making mops from the green/brown camouflage mixes, which look a bit more like natural plants.

Many rivulus produce the majority of their eggs high in the mops and seek the tightest locations near the ties beneath the float. I pick eggs on a daily basis, and they take two to three weeks to hatch. Most species I have worked with produce small numbers of eggs daily, but over time that adds up to large numbers of fry. However, I have picked as many as 100 eggs from R. hartii mops on occasion.

I incubate the eggs in plastic trays that are normally used to hold screws and other workshop items. Some people might be surprised to realize that eggs need water changes, too. I prefer to incubate at warmer temperatures (76° to 78°F), but the eggs will develop fine at lower temperatures—it will just add a few days to the incubation period.

A couple of species need special treatment to get them to hatch. In some species there appears to be some mechanism that requires pressure on the eggs in order for the fry to be able to break free of the surrounding casing.

R. sp. PAN 05-260 is a very good example. I have to admit I stole this technique from a South American annual breeder, Dan Katz, who uses it to force-hatch annual eggs. To get the eggs to pop, I place them into a glass jar, fill it with water, and screw the top on loosely. I then place it at the bottom of a 30-gallon garbage container that holds prepared water. The eggs usually hatch in a few hours.

Initially, fry are kept in a small container—either a half-filled 2½-gallon tank or trays measuring 10 x 6 x 2 inches. I add Java moss and some hornwort to ensure the fry have a constant source of infusoria. One hundred percent water changes daily is by far the optimum choice for fry tanks, but few people have the time or desire to do that.

Fry are fed as often as possible, at least two to three times daily with baby brine shrimp, microworms, and vinegar eels. If you have paramecia, they make a great starter food. The young fish grow reasonably fast the first few months, but then the growth rate slows. Usually when the fry reach an inch, they can be switched to your regular foods.

At this point, the young fish are moved to larger quarters, mostly 20- to 30-gallon tanks about half filled with water. As the fish mature, the water level is raised. Again, my optimal choice is to have the tanks as heavily filled with plants as possible. The young adults can remain in these containers until pairs are separated for future breeding purposes. The others will spend the rest of their lives in these tanks if not passed on to hobbyists or sold.

A Rivulus Wish List

Rivulus xiphidius

When R. xiphidius first appeared in the late 1970s, it was the single most distinctive fish to come into the hobby in many years. A thick black band runs the length of the fish from just behind the eye through the caudal fin. An electric blue line borders above the entire black band. The rest of the tail is a bright orange. The same pattern seen on the tail is also found in the anal and pectoral fins. Females are heavily spotted with light blue and display a fairly distinct dark line running the length of the body.

R. xiphidius is an incredible leaper and will find the tiniest crack to escape. Successful breeders use a variety of similar techniques, but most find very few eggs on a daily basis. A well-planted tank and live foods are almost mandatory. The best technique is to use a single pair. Additional females will eat the eggs left in the mops and, more than likely, so will the parents. They require constant monitoring, and fry need to be removed as soon as they are spotted. Paramecia make a huge difference in the survival rate. R. xiphidius will live for more than three years.

Rivulus mahdiaensis

If R. xiphidius was a surprise to killie hobbyists, then R. mahdiaensis was a shock. Based on both color and finnage, you would assume it to be a West African Aphyosemion. It is a light brown color with several alternating rows of electric-blue and bright-red spots running the length of the body. Both the top and bottom edges of the tail are extended, giving the fish a double-sword appearance. This is one of the smaller species of Rivulus.

Breeding R. mahdiaensis is not difficult using standard mop-spawning techniques; however, the primary reason for its scarcity is an overwhelming number of male offspring. This is one of those fish where water chemistry may affect the sex of the fry—a lower pH produces more females. They need a variety of live foods to keep them in the best breeding condition.

R. mahdiaensis can be raised in groups, but males that are separated and brought back together will fight.

Rivulus obscurus

This is a particularly fascinating fish. I have had them one time only and would love to obtain the species again. While it may already be obvious due to the name, the coloring of the fish is a bit obscure. The body is generally a mixture of browns overlaid by a pattern of light-blue markings and some red spots. These are not bright. The outstanding feature is the tail. The lower third is bright orange and above is black with a brilliant blue line running through. The females are primarily brown with darker brown bands and spotting.

R. obscurus lives near the surface and has developed its jumping skills to very high levels. I have found that this fish often lives out of the water and will attach itself to the side of the tank above the water for hours. The first time I saw this behavior, I nearly had a heart attack thinking they had escaped the tank and were on the floor somewhere. As soon as I removed the cover, they dropped back into the water to hide among the plants. I suspect in nature this is a defense mechanism that allows the fish to escape predators. There are other rivulus who have similar behaviors, but I have never seen it as developed as it is in R. obscurus.

I’ve never had good success in reproducing R. obscurus, and Frans Vermeulen (the photographer for this article) reports it is difficult. His success has come from shallow tanks with a heavy cover of Riccia. Fry are removed from the tank as they are found. Frans uses peat on the bottom of his tank, and it contains a spawning mop.

Rivulus agilae

For many years, R. agilae was considered the most colorful rivulus. It is certainly a beautiful fish, but in today’s hobby it no longer holds the top spot. There are a huge number of color variations and populations. Each is distinctive, but a good rule of thumb in describing this species is to say that the body displays a light-blue side that fades into orange in the lower and/or back third. In some populations, the back half of the fish can appear to have blue vertical bands.

This is one of the easier species to breed and maintain. Standard breeding techniques work quite well, and several eggs a day will be deposited by well-fed pairs. Fry accept brine shrimp nauplii immediately.

Rivulus cryptocallus

This is one of the easiest rivulus to keep and breed, but it is only found on the island of Martinique in the Caribbean. I found this fish in nearly every coastal estuary where fresh water entered the ocean. There were literally thousands of these fish and each location I looked at showed population variations from very brightly colored to very dull.

R. cryptocallus can grow fairly large—over 4 inches. The body of the fish is an olive to light brown and heavily overlaid with light blue. Some exhibit red spots while others do not. I have maintained this species since the early 1990s and rarely pick eggs. A typical mop-spawners setup works fine for them. Many will grow up in the same tank as the parents.

Rivulus christinae

Another new species, this is a striking fish with a light-blue body and several horizontal rows of red spots running the length of it. This is another of the easier species to breed. It produces several eggs a day. Fry take brine shrimp immediately and grow relatively fast.

Rivs

As new species are discovered and the taxonomy is sorted out, there will probably be more genera split out from Rivulus, as Kryptolebias already has been. Hobbyists, however, often refer to the whole group as “rivulus,” or even “rivs.”

Kryptolebias caudomarginatus

The genus Kryptolebias was erected in 2004 and is considered to be a more ancient group than Rivulus. The distribution of this group leads to consideration of how widespread this group may be and how many more species may be uncovered in the future.

It’s actually difficult to believe a basically black-and-white fish could be this distinctive and lovely, but K. caudomarginatus is. It has a light-olive upper body that fades downward into a pale light blue. A series of large black spots or irregular vertical bands run the length of the body. The back third of the caudal and anal fins are solid black. All in all, this is a truly nice-looking fish.

This fish is tough to breed successfully. Males are vicious, and if they do not get their way they have no hesitation in beating up or killing their mates. Some pairs do find a way to cohabitate and will produce large eggs. Fry seem to do well together and will most likely remain tolerant of each other if kept together. Once separated, they prefer their solitude and are likely to attack any of their own gender if introduced.

Kryptolebias (Rivulus) marmoratus

No article about rivulus would be complete without some mention of the original ugly brown fish. There is nothing boring about K. marmoratus’ lifestyle, however. It is found in and around mangrove biotopes throughout the coastal areas of the Caribbean from Florida through Texas and down into Central America, and is a salt- to brackish-water fish that has never been found in fresh water. It is the only known truly hermaphroditic vertebrate—it fertilizes its own eggs. No female has ever been found, though under certain conditions a few males will appear in the population along with the hermaphrodites.

This fish is very aggressive toward its own species and will kill any competition. In nature they are loners that tend to live in crab holes, rotting mangrove logs, and in layers of wet leaves. They can live up to 60 days between layers of wet leaves. They travel over damp ground by flipping from spot to spot and have been found living in the leaves of mangroves on branches overhanging the water.

Breeding is pretty basic. Place a single fish in a tank with a mop and you should find eggs sooner or later.

A Great Group of Killies

With some species being very easy to breed and maintain and others posing a challenge for even the advanced aquarist, rivulus are an interesting group of fish for aquarists of all levels of expertise.

Over Memorial Day weekend (May 28–30) the American Killifish Association will be holding its annual convention in Cleveland, Ohio. There should be many pairs of these and other killifish available for sale at the show and in the auction. For more information, visit www.aka.org.